Soldiers are clamping down on people trying to travel to Liberia's capital from rural areas hard-hit by the Ebola virus after the president declared a national state of emergency.

Reports have emerged of families hiding sick relatives at home and of abandoned bodies being left in the streets.

Similar efforts were under way in eastern areas of neighbouring Sierra Leone after officials there launched Operation Octopus to try to keep those with Ebola in isolation.

While the outbreak has now reached four countries, Liberia and Sierra Leone account for more than 60% of the deaths, according to the World Health Organisation.

The outbreak that emerged in March has claimed at least 932 lives.

In announcing the 90-day state of emergency, Liberian president Ellen Johnson Sirleaf said the worst recorded Ebola outbreak in history requires "extraordinary measures for the very survival of our state and for the protection of the lives of our people".

"Ignorance, poverty, as well as entrenched religious and cultural practices continue to exacerbate the spread of the disease, especially in the counties," she said.

She warned that some civil liberties could be suspended as needed and soldiers are already restricting movements on the roads to the capital, Monrovia, witnesses said.

Some soldiers were deployed to the crossroads town of Klay about 25 miles (40 kilometres) west of Monrovia in an effort to stop people from three Ebola-infected counties from coming closer to the capital.

Yet even as authorities tried to keep more people from reaching Monrovia, the capital has already been hard-hit by the virus.

National Health Workers Association president Joseph Tamba said the state of emergency is necessary. But he said people should have been given advance notice to buy food ahead of the movement restrictions.

Previous outbreaks of the Ebola virus were limited to parts of Congo and Uganda, far from the capital cities in those countries.

The World Health Organisation is holding talks this week on whether to declare an international health emergency.

In Zimbabwe, president Robert Mugabe is considering withdrawing his country's soldiers, police and prison officers serving as UN peacekeepers in Liberia because of the outbreak, the state-run Herald newspaper reported.

"We must not expose ourselves unnecessarily," he was quoted as saying. Peacekeepers are in Liberia until elections in 2017.

In Sierra Leone, President Ernest Bai Koroma has said all Ebola victims must now be buried near where they died to minimise exposure while transporting highly contagious corpses.

"All such burials should be reported to the Ministry of Health and Sanitation, which will give necessary clearance," the order stated. "Death certificates must be issued before burials."

WHO spokesman Gregory Hartl said: "This outbreak, because of its size and its geographical extent, certainly merits an extraordinary response and we know countries have announced they must take extraordinary measures, so that is understandable from a public health perspective."

Liberian Information Minister Lewis Brown vowed stringent surveillance at the country's international airport where many flights have been cancelled because of the outbreak.

"We are facing a threat of the greatest proportion," he said. "No one, absolutely no one will be allowed to enter or leave our country with temperature above normal."

The unprecedented measures came after a man with Ebola in Liberia boarded a flight and ended up in Nigeria, where a nurse who treated him is now dead from the disease and several others are infected. The traveller also died.

Experts warned that extreme measures risk driving patients and their families further underground out of fear. The best way to track possible exposures is through community leaders who are known and respected, they say.

"Nothing replaces that sense of community trust. So if you come in with brute force, things might appear to be working but it could be pushing people out where they can't control the borders and the disease gets out," said Dr David Heymann, professor at London School of Hygiene and Tropical Medicine.

Meanwhile, it has emerged that Africans battling to contain the spread of Ebola would have to wait months before a potentially life-saving experimental drug used on two Americans could even be manufactured.

Even if the experimental drug ZMapp is manufactured in large quantities, its ability to treat Ebola is unproven and no commitment has been publicly made to provide it to Africa.

The health minister of Nigeria, one of the four countries where Ebola has broken out, told a news conference in Washington that he had asked the US Centres for Disease Control and Prevention about access to the drug.

But a CDC spokesman said yesterday that "there are virtually no doses available".

Some people in affected countries have already wondered why the drug was not offered to any infected people in Africa.

Dr Anthony Fauci, of the National Institutes of Health, said the manufacturer has told the US government it would take two to three months to produce even "a modest amount".

"We don't even know if it works," he stressed.

US president Barack Obama has pledged to help "nip as early as possible any additional outbreaks of the disease".

"And then during the course of that process, I think it's entirely appropriate for us to see if there are additional drugs or medical treatments that can improve the survivability of what is a very deadly and obviously brutal disease," he said.

Nouridine Sow, a sociology professor at the Universal Institute of Guinea, said: "This shows simply that white patients and black patients do not have the same value in the eyes of world medicine."

The outbreak was first seen in Guinea and has spread to Sierra Leone, Liberia and Nigeria, killing almost 1,000 people since March.